Apparatuses and methods for controlling refresh operations

ABSTRACT

An apparatus includes a first word line, a second word line and a control. The second word line is contiguous to the first word line. The control circuit includes a first defective address storing circuit and a first detection circuit. The first defective address storing circuit stores first enable information along with first defective address. The first enable information indicates whether or not the second word line is functional. The first detection circuit provides a first signal when the first word line is accessed. The first signal indicates whether or not the second word line is functional. The control circuit activates the second word line when the first signal indicates that the second word line is functional and does not activate the second word line when the first signal indicates that the second word line is not functional.

PRIORITY

This application is a continuation of U.S. patent application Ser. No.14/713,942, filed May 15, 2015, and issued as U.S. Pat. No. 9,922,694 onMar. 20, 2018, which claims the benefit of priority from Japanese PatentApplication No. 2014-105197 filed on May 21, 2014. The aforementionedapplications and patent are incorporated herein by reference, in theirentirety, for any purpose.

BACKGROUND

Since a DRAM (Dynamic Random Access Memory), which is a typicalsemiconductor memory device, stores information by the electric chargeaccumulated in a cell capacitor, the information is lost unless arefresh operation is periodically carried out. Therefore, JapanesePatent Application Laid-Open No. 2011-258259 discloses that a controldevice, winch controls the DRAM, periodically issues a refresh command,which orders the refresh operation, to the DRAM. The refresh command isissued from the control device at a frequency that all word lines arecertainly refreshed once in the period of one refresh cycle (forexample, 64 msec).

However, depending on the history of access to memory cells, theinformation retention time of predetermined memory cells are reduced insome cases. Then, when the information retention time of thepredetermined memory cells is reduced to less than the one refreshcycle, there has been a risk that part of the information may be losteven if the refresh command is issued at the frequency that all the wordlines are refreshed once in the period of one refresh cycle.

In order to solve such a problem, recently, a technique which restoresthe electric charge with respect to theinformation-retention-time-reduced memory cells by utilizing the historyof access to the memory cells has been studied. This technique is calledtarget-row refresh.

Herein, in a case in which a defective word line is replaced by aredundant word line, erroneous operations may occur unless thetarget-row refresh operation is carried out in consideration of that.This is for a reason that actually-used (functional) redundant wordlines and unused (non-functional) redundant word lines are mixed in theredundant word lines. More specifically when the unused redundant wordline is selected in a target-row refresh operation, for example, ifmicro short-circuit is present at the unused redundant word line or thememory cells connected thereto, there has been a risk that other datamay be broken.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a semiconductordevice according to an embodiment of the present invention.

FIG. 2 is a drawing showing address allocation of normal word lines andredundant word lines according to an embodiment of the presentinvention.

FIG. 3 is a block diagram showing a configuration of a row controlcircuit according to an embodiment of the present invention.

FIG. 4 is a circuit diagram of a state-signal generation circuitaccording to an embodiment of the present invention.

FIG. 5 is a timing diagram for explaining the operations of thestate-signal generation circuit according to an embodiment of thepresent invention.

FIG. 6 is a circuit diagram of an address control circuit according toan embodiment of the present invention.

FIG. 7 is a circuit diagram of a logic circuit according to anembodiment of the present invention.

FIG. 8 is a circuit diagram of a boundary detection circuit according toan embodiment of the present invention.

FIG. 9 is a block diagram showing a configuration of an addressdetection circuit and a usage detection circuit according to anembodiment of the present invention.

FIG. 10 is a circuit diagram of a fuse set according to an embodiment ofthe present invention.

FIG. 11 is a circuit diagram of a detection circuit according to anembodiment of the present invention.

FIG. 12 is a circuit diagram of a detection circuit according to anembodiment of the present invention.

FIG. 13 is a circuit diagram of a detection circuit according to anembodiment of the present invention.

FIG. 14 is a circuit diagram of a boundary detection circuit accordingto an embodiment of the present invention.

FIG. 15 is a circuit diagram of an address conversion circuit containedin a row predecoder according to an embodiment of the present invention.

FIG. 16 is a circuit diagram of an address conversion circuit containedin the row predecoder according to an embodiment of the presentinvention.

FIG. 17 is a timing chart showing target-row refresh operations in acase 1.

FIG. 18 is a first timing chart showing target-row refresh operations ina case 2.

FIG. 19 is a second timing chart showing target-row refresh operationsin a case 2.

FIG. 20 is a timing chart showing target-row refresh operations in acase 3.

FIG. 21 is a first timing chart showing target-row refresh operations ina case 4.

FIG. 22 is a timing chart showing target-row refresh operations in thecase 2.

DETAILED DESCRIPTION

Certain details are set forth below to provide a sufficientunderstanding of embodiments of the invention. However, it will be clearto one skilled in the art that embodiments of the invention may bepracticed without these particular details. Moreover, the particularembodiments of the present invention described herein are provided byway of example and should not be used to limit the scope of theinvention to these particular embodiments. In other instances, wellknown circuits, control signals, timing protocols, and softwareoperations have not been shown in detail in order to avoid unnecessarilyobscuring the invention.

FIG. 1 is a block diagram showing a configuration of a semiconductordevice 10 according to the embodiment of the present invention.

The semiconductor device 10 according to the present embodiment is, forexample, a DDR4 (Double Data Rate 4) type DRAM integrated on a singlesemiconductor chip and carries out read operations and write operationswith respect to a memory cell array 20 based on external clock signalsCLK and command/address signals C/A, which are input from an externalcontroller. Data signals DATA which are read from the memory cell array20 in the read operations are output to outside via a data controlcircuit 21 and a data input/output circuit 22. Data signals DATA whichare input from outside in the write operations are written to the memorycell array 20 via the data input/output circuit 22 and the data controlcircuit 21.

As shown in FIG. 1, the external clock signals CLK and the externalcommand address signals C/A are input to an input receiver circuit 23.The external clock signals CLK input to the input receiver circuit 23are supplied to a clock generation circuit 24, thereby generatinginternal clock signals iCLk. The internal clock signals iCLK are used astiming signals which define operation timing of the semiconductor device10 according to the present embodiment.

The external command/address signals C/A input to the input receivercircuit 23 are supplied to a command decoder 25, an address latchcircuit 26, and a mode register 27 as internal command/address signalsiC/A. The command decoder 25 decodes command signals, which arecontained in the internal command/address signals iC/A, therebygenerating various control signals. Examples of the control signalsgenerated by the command decoder 25 include active signals iACT,read/write signals iW/R, refresh signals iREF, precharge signals iPRE,address selection signals ADDSEL, and column control signals CCTL.

The active signal iACT is activated when the internal command/addresssignal iC/A is indicating an active command. When the active signal iACTis activated, a row address contained in the internal command/addresssignal iC/A is latched by the address latch circuit 26. The row addressRADD1 latched by the address latch circuit 26 is supplied to a rowcontrol circuit 28. Based on the row address RADD1, the row controlcircuit 28 selectively activates any of a plurality of normal word linesWL and a plurality of redundant word lines RWS contained in the memorycell array 20. FIG. 1 shows a single normal word line WL and a singleredundant word line RWL in the memory cell array 20. However, inpractice, many normal word lines WL and many redundant word lines RWLare provided.

The read/write signal iW/R is activated when the internalcommand-address signal iC/A is indicating a read command or a writecommand. When the internal command/address signal iC/A is indicating theread command or the write command, the column control signal CCTL isalso activated. When the read/write signal iW/R is activated, a columnaddress contained in the internal command/address signal iC/A is latchedby the address latch circuit 26. The column address CADD latched by theaddress/latch circuit 26 is supplied to a column control circuit 29.When the column control signal CCTL is activated, the column controlcircuit 29 selectively connects any of a plurality of bit lines BL,which are contained in the memory cell array 20, to a correspondingsense amplifier SA based on the column address CADD. FIG. 1 shows asingle bit line BL in the memory cell array 20. However, in practice,many bit lines BL are provided. The bit lines BL are connected to senseamplifiers SA, and read or write of the data signals DATA is carried outvia the sense amplifiers SA.

More specifically, in a read operation, the data signal DATA is readfrom a normal memory cell MC disposed at the intersection point of theselected normal word line WL and the selected bit line BL or a redundantmemory cell RMC disposed at the intersection point of the selectedredundant word line RWL and the selected bit line BL, and the datasignal DATA is output to outside via the data control circuit 21 and thedata input/output circuit 22.

On the other hand, in a write operation, the data signal DATA input fromoutside is supplied to the memory cell array 20 via the data inputoutput circuit 22 and the data control circuit 21, and the data signalDATA in written to a normal memory cell MC disposed at the intersectionpoint of the selected normal word line WL and the selected bit line BLor a redundant memory cell RMC disposed at the intersection point of theselected redundant word line RWL and the selected bit line BL.

The refresh signal iREF is activated when the internal command/addresssignal iC/A is indicating a refresh command. The refresh signal iREF issupplied to the row control circuit 28, thereby executing a refreshoperation of normal memory cells MC or redundant memory cells RMC.

The precharge signal iPRE is activated when the internal command/addresssignal iC/A is indicating a precharge command. The precharge signal iPREis supplied to the row control circuit 28, thereby deactivating thenormal word line WL or the redundant word line RQL which have beenactivated.

The address selection signal ADDSEL is activated when the internalcommand/address signal iC/A is indicating the active command or therefresh command. The address selection signal ADDSEL is supplied to therow control circuit 28, thereby executing selection of the normal wordline WL or the redundant word line RWL serving as an access target.

Although details will be described later, the row control circuit 28includes many antifuse elements. The antifuse element is an elementwhich is insulated in an initial state and, when subjected to dielectricbreakdown by a connect operation, makes a transition to a conductivestate. When the transition to the conductive state is once made by theconnect operation, the antifuse element cannot be returned again to theinsulated state. Therefore, the antifuse element can be used as anonvolatile and irreversible storage element. The connect operation withrespect to the antifuse element uses a high potential VPPSV and anegative potential VBBSV. The high potential VPPSV and the negativepotential VBBSV is generated by a power-source circuit 30 based on apower-source potential VDD and a ground potential VSS.

In the mode register 27, a parameter indicating an operation mode of thesemiconductor device 10 according to the present embodiment is set. Forexample, if the internal command/address signal iC/A is indicating atarget-row refresh command, a parameter indicating a target-row refreshmode is registered in the mode register 27, and a target-row refreshenable signal TRREN is activated. The target-row refresh enable signalTRREN is supplied to the row control circuit 28.

FIG. 2 is a drawing for explaining address allocation of the normal wordlines WL and the redundant word lines RWL.

As shown in FIG. 2, n+1-bit row addresses of Xn to X0 are assigned tothe normal word lines WL, respectively, and the single normal word lineWL is specified by each row address. Similarly, n+1-bit replacementaddresses of RXn to RX0 are assigned to the redundant word lines RWL,respectively, and the single redundant word line RWL is specified byeach replacement address. The number of the bits of the row address andthe number of the bits of the replacement address are not necessarilythe same. For example, the number of the bits of the replacement addressmay be smaller than the number of the bits of the row address.

The normal word lines WL are arranged in the order of the row addressesfrom the normal word line WL0 of the row address having all of thevalues of bits Xn to X0 being “0” to the normal word line WLN of the rowaddress having all of the values of bits Xn to X0 being “1”. Similarly,the redundant word lines RWL are arranged in the order of thereplacement addresses from the redundant word line RWL0 of thereplacement address having all of the values of bite RXn to RX0 being“0” to the redundant word line RWLN of the replacement address havingall of the values of bits RXn to RX0 being “1”. However, the normal wordlines WL can be provided more than the redundant word lines RWL.

The normal word lines WL0 to WLN and the redundant word lines RWL0 toRWLN are continuously located. Therefore, one direction (upwarddirection shown in FIG. 2) viewed from a boundary B between the lastnormal word line WLN and the first redundant word line RWL0 serves as anormal-word-line area A1 in which the normal word lines WL0 to WLN arelocated, and the opposite direction (downward direction shown in FIG. 2)viewed form the boundary B serves as a redundant-word-line area A2 inwhich the redundant word lines RWL0 to RWLN are disposed.

As shown in FIG. 2, the two normal word lines WL at which only the leastsignificant bit X0 is different among the bits Xn to X0 of the rowaddresses are disposed to be mutually adjacent. Similarly, the tworedundant word lines RWL at which only the least significant bit X0 isdifferent among the bits RXn to RX0 of the replacement addresses arealso disposed to be mutually adjacent, if a defect is present in any ofthe normal word lines WL, a set having not only the normal word line WLthat has the defect but also the different normal word line WL pairedtherewith is replaced by the redundant word lines RWL. For example, if adefect is present in the normal word line WLK, the paired normal wordlines WLK-1 and WLK are collectively replaced by, for example, theredundant word lines RWLJ-1 and RWLJ. This is for reducing the number ofthe antifuse elements required.

Herein, if access is repeated with respect to a certain normal word lineWL or redundant word line RWL, the information retention time of thenormal memory cells MC or the redundant memory cells RMC connected tothe normal word line WL or the redundant word line RWL adjacent theretoare reduced. With respect to the normal memory cells MC or the redundantmemory cells RMC having the reduced information retention time, theinformation retention time have to be recovered by carrying out atarget-row refresh operation.

For example, access with respect to the normal word line WL2 shown inFIG. 2 is repeated, the information retention time of the normal memorycells MC connected to the normal word lines WL1 and WL3 adjacent theretoare reduced. This case can be solved by executing a target-row refreshoperation with respect to the normal word lines WL1 and WL3. In thiscase (case 1), all of the word lines serving as the targets of thetarget-row refresh operation belong to the normal-word-line area A1.

If access is repeated with respect to the normal word line WLN shown inFIG. 2, the information retention time of the normal memory cells MCconnected to the normal word line WLN-1 adjacent thereto and theredundant memory cells RMC connected to the redundant word line RWL0 arereduced. This case can be solved by executing the target-row refreshoperation with respect to the normal word line WLN-1 and the redundantword line RWL0. In this case (case 2), one of the word lines serving asthe targets of the target-row refresh operation belongs to thenormal-word-line area A1, and the other one belongs to theredundant-word-line area A2.

Furthermore, if access with respect to the redundant word line RWL0shown in FIG. 2 is repeated, the information retention time of thenormal memory cells MC connected to the normal word line WLN connectedthereto and the redundant memory cells RMC connected to the redundantword line RWL1 are reduced. This case can be solved by executing thetarget-row refresh operation with respect to the normal word line WLNand the redundant word line RWL1. Also in this case (case 3), one of theword lines serving as the targets of the target-row refresh operationbelongs to the normal-word-line area A1, and the other one belongs tothe redundant-word-line area A2.

If access with respect to the redundant word line RWL2 shown in FIG. 2is repeated, the information retention time of the redundant memorycells RMC connected to the redundant word lines RWL1 and RWL3 adjacentthereto are reduced. This case can be solved by executing the target-rowrefresh operation with respect to the redundant word lines RWL1 andRWL3. In this case (case 4), all of the word lines serving as thetargets of the target-row refresh operation belong to theredundant-word-line area A2.

If the normal-word-line area A1 and the redundant-word-line area A2 arecontinuously provided to be mutually adjacent in this manner, thetarget-row refresh operations have to be executed in consideration ofthe above described four cases. Although this point will be describedlater in detail, in order to correctly execute the target-row refreshoperations in any of the four cases, the present embodiment includes thecircuits which are necessary for correctly carrying out the target-rowrefresh operations in a case of occurrence of any of the case 1 to thecase 4.

First, regarding the case 1 to the case 4, the circuits (address controlcircuit, address conversion circuit 43, 44) are provided for generatingthe addresses of the normal word lines WL or the redundant word linesRWL serving as the targets of target-row refresh are provided. Moreover,regarding the case 2, the circuit (boundary detection circuit 36) isprovided for detecting that the normal word line WLN which is theclosest to the redundant-word-line area A2 in the normal-word-line areaA1 serves as a cause of the target-row-refresh operation. Similarly,regarding the case 3, the circuit (boundary detection circuit 40) isprovided for detecting that the redundant word line RWL0 which is theclosest to the normal-word-line area A1 serves as a cause of thetarget-row refresh operation. Furthermore, regarding the case 2 and thecase 4, the circuit (usage detection circuit 37) is provided fordetecting whether the redundant word line(s) RWL to be subjected toexecution of the target-row refresh operation is actually used(functional) or not (non-functional).

FIG. 3 is a block diagram showing a configuration of the row controlcircuit 28.

As shown in FIG. 3, the row control circuit 28 includes a selector 31,which generates a row address RADD2. The selector 31 receives the rowaddress RADD1 supplied from the address latch circuit 26, a row addressRADD1 a supplied from an address control circuit 32, and a row addressRADD1 b supplied from a refresh address counter 33 and outputs any ofthe row addresses RADD1, RADD1 a, and RADD1 b as the RADD2.

The selection by the selector 31 is carried out based on the addressselection signal ADDSEL and state signals TRRST2 and TRRST3.Specifically, if the address selection signal ADDSEL is indicating anissuance of an active command and both of the state signals TRRST2 andTRRST3 are at a low level (non-active level), the selector 31 selectsthe row-address RADD1 and outputs this as the row address RADD2. If theaddress selection signal ADDSEL is indicating an issuance of an activecommand and at least one of the state signals TRRST2 and TRRST3 is at ahigh level (active level), the selector 31 selects the row address RADD1a and outputs this as the row address RADD2.

Furthermore, if the address selection signal ADDSEL is indicating anissuance of a refresh command, the selector selects the raw addressRADD1 b regardless of the state signals TRRST2 and TRRST3 and outputsthis as the row address RADD2.

The state signals TRRST2 and TRRST3 are generated by a state-signalgeneration circuit 34.

FIG. 4 is a circuit diagram of the state-signal generation circuit 34.

As shown in FIG. 4, the state-signal generation circuit 34 includes aSR-type flip-flop circuit 51 and D-latch-type flip-flop circuits 52 to54. The flip-flop circuit 51 is set by the target-row refresh enablesignal TRREN and is reset by the output signal of the flip-flop circuit54. The flip-flop circuit 51 provides a state signal TRRST1 to resetnodes of the flip-flop circuits 52 to 54. The reset nodes of theflip-flop circuits 52 to 54 employ negative logic. Therefore, if thetarget-row refresh enable signal TRREN is activated, the reset state ofthe flip-flop circuits 52 to 54 is cancelled.

The flip-flop circuits 52 to 54 are cascade-connected in this order, andall of them carry out latch operations in synchronization with theprecharge signal iPRE. An input node of the flip-flop circuit 52 isfixed to a high level. The output signal of the flip-flop circuit 52 isused as the state signal TRRST2, and the output signal of the flip-flopcircuit 53 is used as the state signal TRRST3. The output signal of theflip-flop circuit 54 resets the flip-flop circuit 51 as described above.

FIG. 5 is a timing diagram for explaining the operations of thestate-signal generation circuit 34.

As shown in FIG. 5, when the target-row refresh enable signal TRREEN isactivated, first, the state signal TRRST1 is activated. Then, the statesignal TRRST2 is activated to the high level in response to the firstprecharge signal iPRE, the state signal TRRST3 is activated to the highlevel in response to the second precharge signal iPRE, and all of thestate signals TRRST1 to TRRST3 are deactivated to the low level inresponse to the third precharge signal iPRE.

FIG. 6 is a circuit diagram of the address control circuit 32.

As shown in FIG. 6, the address control circuit 32 includes an inversioncircuit 61, which inverts the least significant bit X0 of the rowaddress RADD1; an address adder circuit 62, which increments moresignificant bits Xn to X1 excluding the least significant bit X0 of therow address RADD1 by only 1 bit; and an address subtracter circuit 63,which decrements the more significant bits Xn to X1 excluding the leastsignificant bit X0 of the row address RADD1 by only 1 bit. The rowaddress RADD1 and the output signals from the circuits 61 to 63 areinput to a selector 64.

If a calculation enable signal CALEN is activated to the high level, theaddress adder circuit 62 and the address subtracter circuit 63 executesthe increment or the decrement and outputs the addition result or thesubtraction result of the more significant bits Xn to X1 of the rowaddress RADD1. If the calculation enable signal CALEN is deactivated tothe low level, the address adder circuit 62 and the address subtractercircuit 63 output the more significant bits Xn to X1 of the row addressRADD1 with no change without carrying out the increment or thedecrement. The calculation enable signal CALEN is generated by a logiccircuit 65 shown in FIG. 6. If the state signal TRRST3 is at the highlevel and all of flag signals FLG1 to FLG3 are deactivated to the lowlevel, the logic circuit 65 activates the calculation enable signalCALEN to the high level.

If a selection signal SEL2 is activated, the selector 64 generates therow address RADD1 a by synthesizing the more significant bits Xn to X1of the row address RADD1 and the least significant bit X0 having a valueinverted by the inversion circuit 61. If a selection signal SEL3 isactivated, the selector 64 outputs the row address RADD1 a bysynthesizing the more significant bits Xn to X1 of the row addressRADD1, which are output from the address adder circuit 62 or the addresssubtracter circuit 63, and the least significant bit X0 having a valueinverted by the inversion circuit 61. Specifically, in a case in whichthe calculation enable signal CALEN is activated to the high level, ifthe least significant bit X0 of the row address RADD1 is “1”, the valueobtained by synthesizing the more significant bits Xn to X1 of the rowaddress RADD1 incremented by the address adder circuit 62 with the leastsignificant bit X0 having the value inverted by the inversion circuit 61is output as the row address RADD1 a; and, if the least significant bitX0 of the row address RADD1 is “0”, the value obtained by synthesizingthe more significant bits Xn to X1 of the row address RADD1 decrementedby the address subtracter circuit 63 with the least significant bit X0having the value inverted by the inversion circuit 61. If thecalculation enable signal CALEN is deactivated to the low level, both ofthe address adder circuit 62 and the address subtracter circuit 63output the more significant bits Xn to X1 of the row address RADD1 withno change; therefore, the row address RADD1 a output by the selector 64is the row address in which only the least significant bit X0 isinverted among Xn to X0 of the row address RADD1.

Furthermore, OR gate circuits 66 are inserted to an output node of theselector 64. In FIG. 6, the OR gate circuit 66 is shown by one symbolmark. However, in practice, the OR gate circuits 66 are insertedrespectively for the bits Xn to X0 of the row address RADD1 a. The flagsignal FLG2 is input to the OR gate circuits 66. Therefore, if the flagsignal FLG2 is activated to the high level, the values of the rowaddress RADD1 a are forced to be maximum values, in other words, all ofthe values Xn to X0 become “1”. The word line at which all of the valuesXn to X0 are “1” corresponds to the normal word line WLN shown in FIG.2.

The selection signals SEL2 and SEL3 are generated by a logic circuit 60shown in FIG. 7. The operations of the logic circuit 60 are shown inFIG. 5. The selection signal SEL2 is activated to the high level in theperiod from the first precharge signal iPRE to the second prechargesignal iPRE, and the selection signal SEL3 is activated to the highlevel in the period from the second precharge signal iPRE to the thirdprecharge signal iPRE.

Therefore, as shown in FIG. 5, in a period T1 between activation of thetarget-row refresh enable signal TRREN and generation of the firstprecharge signal iPRE, the row address RADD1 is output with no change asthe row address RADD2. Then, in a period T2 between generation of thefirst precharge signal iPRE and generation of the second prechargesignal iPRE, the row address RADD1 a having the least significant bit X0inverted by the inversion circuit 61 is output as the row address RADD2.Then, in a period T3 between generation of the second precharge signaliPRE and generation of the third precharge signal iPRE, if thecalculation enable signal CALEN is at the high level, the row addressRADD1 a incremented by the address adder circuit 62 or the row addressRADD1 a decremented by the address subtracter circuit 63 is output asthe row address RADD2. If the calculation enable signal CALEN is at thelow level, as well as the second one, the row address RADD1 a having theinverted least significant bit X0 is output as the row address RADD2.

However, the above described operations are the operations of the casein which all of the flag signals FLG1 to FLG3 are deactivated to the lowlevel. If any of the flags FLG1 to FLG3 is at the high level, theoperations different from those described above are executed. They willbe described in detail later.

Returning to FIG. 3, the row address RADD2 output from the selector 31is input to an address detection circuit 35 and the boundary detectioncircuit 36. The least significant bit X0 of the row address RADD2 issupplied also to the usage detection circuit 37.

The address detection circuit 35 judges whether the row address RADD2 isindicating the address of the normal word line WL that has a detect ornot. The boundary detection circuit 36 judges whether the row addressRADD2 is indicating the address of the normal word line WLN positionedat the end of the normal-word-line area A1 or not. The usage detectioncircuit 37 is a circuit which judges whether the redundant word line RWLadjacent to the redundant word line RWL serving as the target of thetarget-row refresh is used or not.

FIG. 8 is a circuit diagram of the boundary detection circuit 36.

As shown in FIG. 8, the boundary detection circuit 36 includes an ANDgate circuit 78, which receives the bits X0 to Xn constituting the rowaddress RADD2, and a latch circuit 79, which latches the output of theAND gate circuit 78 in response to the state signal TRRST2. By virtue ofsuch a configuration, if all of the values of the bits Xn to X0constituting the row address RADD2 are “1”, the flag signal FLG1, whichis a boundary detection signal, is activated to the high level inresponse to the state signal TRRST2. More specifically, if the rowaddress RADD2 corresponding to the normal word line WLN shown in FIG. 2is input, the flag signal FLG1 becomes the high level in response to thestate signal TRRST2.

FIG. 9 is a block diagram showing a configuration of the addressdetection circuit 35 and the usage detection circuit 37.

As shown in FIG. 9, the address detection circuit 35 includes aplurality of fuse sets FSET0 to FSETm (defective address storingcircuits). The fuse sets FSET0 to FSETm can store defective addresses,respectively. If the input row address RADD2 matches the storeddefective address, the corresponding one of the hit signals HIT0 to HITmis activated to the high level.

The hit signals HIT0 to HITm are supplied to an encoder 38. If any ofthe hit signals HIT0 to HITm is activated, the encoder 38 generates arow address RADD3 by encoding the hit signal and supplies the rowaddress to a selector 39.

If none of the hit signals HIT0 to HITm is activated, the selector 39selects the row address RADD2 and supplies this to a row predecoder 41.On the other hand, if any of the hit signals HIT0 to HITm is activated,the selector 39 selects the row address RADD3 and supplies this to therow predecoder 41.

The row predecoder 41 controls a row decoder 42 by predecoding the rowaddress RADD2 or RADD3. The row decoder 42 includes a normal-word-linerow decoder and a redundant-word-line row decoder and selects the normalword line WL or the redundant word line RWL by completely decoding therow address RADD2 or RADD3. Specifically, if the selector 39 isselecting the row address RADD2, any of the normal word line WL isselected by the row decoder 42. On the other hand, if the selector 39 isselecting the row address RADD3, any of the redundant word line RWL isselected by the row decoder 42. The row predecoder 41 and the rowdecoder 42 constitute a word-line drive circuit.

FIG. 10 is a circuit diagram of the fuse set FSETk.

As shown in FIG. 10, the fuse set FSETk includes antifuse circuits AFC1to AFCn (first storage unit) and an enable circuit ENC (second storageunit). The antifuse circuits AFC1 to AFCn are the circuits respectivelycorresponding to the bits X1 to Xn of the address RADD2, and each ofthem includes the antifuse element AF, a load circuit 71, and acomparison circuit 72. The antifuse circuit corresponding to the leastsignificant bit X0 of the address RADD2 is not provided.

The antifuse element AF is an element which is insulated in an initialstate and, when subjected to dielectric breakdown by a connectoperation, makes a transition to a conductive state. The conductivestate of the antifuse element AF is detected by the load circuit 71. Ifit is in an insulated state, the output of the load circuit 71 becomesthe high level. If it is in a conductive state, the output of the loadcircuit 71 becomes the low level. The output of the load circuit 71 isinput to the comparison circuit 72 and is compared with the logic levelof the corresponding bit of the row address RADD2. If both of them matchwith each other, the comparison circuit 72 causes an output signal C1 toCn to be the high level. Reversely, if both of them do not match witheach other, the comparison circuit 72 causes the output signal C1 to Cnto be the low level.

The enable circuit ENC includes the antifuse element AF, a load circuit71, and an inverter circuit 73. The enable circuit ENC is a circuitwhich indicates whether the fuse set FSETk is used or not. If used, theantifuse element AF of the enable circuit ENC is connected. As a result,if fuse set FSETk is used, an enable signal AFENk output from the enablecircuit ENC becomes the high level.

The output signals C1 to Cn of the antifuse circuits AFC1 to AFCn andthe enable signal AFENk are input to an AND gate circuit 74, which is acomparison unit. As a result, on the condition that the enable signalAFENk is activated to the high level, the hit signal HITk is activatedwhen the bits X1 to Xn of the input row address RADD2 and the defectiveaddress stored in the fuse set FSETk completely match with each other.

The other fuse sets FSET0 to FSETm also have similar circuitconfigurations and generate the hit signals HIT0 to HITm, respectively.

The hit signals HIT0 to HITm are input to the encoder 38 as describedabove and are input also to the usage detection circuit 37 and an ORgate circuit 77. As shown in FIG. 9, the usage detection circuit 37 hasdetection circuits DC0 to DCm corresponding to the fuse sets FSET0 toFSETm. If at least any one of the hit signals HIT0 to HITm is at thehigh level, the OR gate circuit 77 activates a Hit signal HITa to thehigh level. The hit signal HITa is supplied to the detection circuit DC0of the usage detection circuit 37.

FIG. 11 is a circuit diagram of the detection circuit DCk.

As shown in FIG. 11, the detection circuit DCk includes an AND gatecircuit 81, which receives the enable signal AFENk−1 and the invertedsignal of the least significant bit X0 of the row address RADD2; an ANDgate circuit 82, which receives the enable signal AFENk+1 and the leastsignificant bit X0 of the row address RADD2; an OR gate circuit 83,which receives the output signals of the AND gate circuits 81 and 82;and an AND gate circuit 84, which receives the hit signal HITk and theoutput signal of the OR gate circuit 83. By virtue of thisconfiguration, in a case in which the hit signal HITk is activated tothe high level, if the least significant bit X0 of the row address RADD2is at the low level and the enable signal AFENk−1 is at the high level,or if the least significant bit X0 of the row address RADD2 is at thehigh level and the enable signal AFENk+1 is at the high level, a usagedetection signal USEk is activated to the high level.

The usage detection signal USEk indicates that the redundant word lineRWL different from and adjacent to the redundant word line RWL to beaccessed based on the hit signal HITk is used. The other detectioncircuits DC1 to DCm-1 excluding the detection circuits DC0 and DCmpositioned at both ends also have the circuit configurations similar tothat of the detection circuit DCk shown in FIG. 10. More specifically,the AND gate circuit 81 receives the enable signal AFEN of the fuse setFSET which is immediately therebefore, and the AND gate circuit 82 isconfigured to receive the enable signal AFEN of the fuse set FSET whichis immediately thereafter.

On the other hand, the detection circuit DC0 corresponding to the fuseset FSET0, in other words, the redundant word line RWL0 adjacent to thenormal word line WLN and the detection circuit DCm corresponding to thefuse set FSETm have the circuit configurations different from that ofthe detection circuit DCk shown in FIG. 11. FIG. 12 is a circuit diagramof the detection circuit DC0.

As shown in FIG. 12, in the detection circuit DC0, OR gate circuits 85and 88 are added, and the AND gate circuits 81 and 82 are replaced byAND gate circuit 86 and 87. The OR gate circuit 88 receives the hitsignal HITa supplied from the OR gate circuit 77 of FIG. 7 and theinverted signal of the flag signal FLG1 and supplies the output thereofto the OR gate circuit 85. The OR gate circuit 85 receives the outputsignal of the OR gate circuit 88 and the hit signal HIT0 and the hitsignal HIT0 and supplies the output thereof to the AND gate circuit 84.The AND gate circuit 86 receives the enable signal AFEN0, the invertedsignal of the hit signal HIT0, and the least significant bit X0 of therow address RADD2. Furthermore, the AND gate circuit 87 receives theenable signal AFEN1, the inverted signal of the flag signal FLG1, andthe least significant bit X0 of the row address RADD2. The otherconfiguration thereof has the same circuit configuration as that of thedetection circuit DCk shown in FIG. 11.

By virtue of this configuration, in a case in which the hit signal HIT0is at the high level and the flag signal FLG1 is at the low level, ifthe least significant bit X0 of the row address RADD2 is at the highlevel and the enable signal AFEN1 is at the high level, a usagedetection signal USE0 is activated to the high level. In a case in whichthe hit signal HITa is at the low level, in other words, all of the hitsignals HIT0 to HITm are at the low level and the flag signal FLG1 is atthe high level, if the least significant bit X0 of the row address RADD2is at the high level and the enable signal AFEN0 is at the high level,the usage detection signal USE0 is activated to the high level. Even inthe case in which the flag signal FLG1 is at the high level, if the hitsignal HITa is at the high level in other words, any of the hit signalsHIT0 to HITm is at the high level, the usage signal USE0 is notactivated.

FIG. 13 is a circuit diagram of the detection circuit DCm.

As show in FIG. 13, the detection circuit DCm is different from thedetection circuit DCk shown in FIG. 11 in a point that one of the inputnodes of the AND gate circuit 82 is fixed to the low level. The otherconfiguration has the same circuit configuration as that of thedetection circuit DCk shown in FIG. 11.

By virtue of this configuration, when the hit signal HITm is activatedto the high level, only if the least significant bit X0 of the rowaddress RADD2 is at the low level and the enable signal AFENm-1 is atthe high level, a detection signal USEm is activated to the high level.

The usage detection signals USE0 to USEm generated in this manner areinput to a latch circuit 76 via an OR gate circuit 75. Since the latchcircuit 76 carries out a latch operation in synchronization with thestate signal TRRST2, if any of the usage detection signals USE0 to USEmis at the high level, the flag signal FLG3 is activated to the highlevel in synchronization with the state signal TRRST2.

FIG. 14 is a circuit diagram of the boundary detection circuit 40.

As shown in FIG. 14, the boundary detection circuit 40 includes a latchcircuit 91, which latches the hit signal HIT0 in synchronization withthe state signal TRRST3; and an AND circuit 92, which receives theoutput signal of the latch circuit 91 and the inverted signal of theleast significant bit X0 of the row address. By virtue of thisconfiguration, if the hit signal HIT0 is at the high level and the leastsignificant bit X0 of the row address is “0”, the flag signal FLG2,which is the boundary detection signal, is activated to the high levelin synchronization with the state signal TRRST3.

FIG. 15 is a circuit diagram of the address conversion circuit 43contained in the row predecoder 41.

As shown in FIG. 15, the address conversion circuit 43 includes an ANDgate circuit 93, which receives the flag signal FLG1, the usagedetection signal USE0, and the state signal TRRST3; and a switch circuit94, which is activated by the output signal of the AND gate circuit 93.If the output signal of the AND gate circuit 93 becomes the high level,the switch circuit 94 activates a switch signal FRC0. If the switchsignal FRC0 is activated, regardless of the values of the row addressesRADD2 and RADD3 input from the selector 39, the redundant word line RWL0is forcibly selected by the row decoder 42. Therefore, if both of theflag signal FLG1 and the usage detection signal USE0 are activated, theredundant word line RWL0 is forcibly selected in response to the statesignal TRRST3.

FIG. 16 is a circuit diagram of the address conversion circuit 44contained in the row predecoder 41.

As shown in FIG. 16, the address conversion circuit 44 includes an ANDgate circuit 95, which receives the inverted signal of the flag signalFLG1, the flag signal FLG3, and the state signal TRRST3; and a switchcircuit 96, which is activated by the output signal of the AND gatecircuit 95. If the output signal of the AND gate circuit 95 becomes thehigh level, the switch circuit 96 activates a switch signal FRC1. If theswitch signal FRC1 is activated, the value of the row address RADD3input from the selector 39 is changed, and among the redundant wordlines RWL adjacent to the redundant word line RWL corresponding to therow address RADD3, the redundant word line RWL in the opposite side ofthe redundant word line RWL at which only the least significant bit RX0of the replacement addresses Rxn to RX0 is different is forciblyselected. Therefore, if the flag signal FLG1 is deactivated and the flagsignal FLG3 is activated, the redundant word line RWL adjacent to theredundant word line RWL corresponding to the row address RADD3 isselected in response to the state signal TRRST3.

The circuit configuration of the semiconductor device 10 according tothe present embodiment is as described above. Next, the operations ofthe semiconductor device 10 according to the present embodiment will beexplained for each case with reference to FIG. 17 to FIG. 22.

[Case 1]

FIG. 17 is a timing chart for explaining the target-row refreshoperations in the case 1. The case 1 is that access has been repeatedwith respect to a normal word line WLi (for example, the normal wordline WL2 shown in FIG. 2), the target-row refresh operations areexecuted with respect to normal word lines WLi−1 and WLi+1 (for example,WL1 and WL3) adjacent thereto. In other words, this is a case in whichall of the word lines serving as the cause of the target-row refreshoperations and the word lines serving as the targets of the target-rowrefresh operations belong to the normal-word line area A1.

In FIG. 17, the address output from the row predecoder 41, in otherwords, the address of the normal word line WL or the redundant word lineRWL which is actually accessed is described as RADD4. This point issimilar also in below FIG. 18 to FIG. 22.

First, if the number of access with respect to the certain normal wordline WLi (for example, the normal word line WL2) exceeds a predeterminedvalue, a target-row refresh command TRR is issued from an externalcontroller (time t11). As a result, the target-row refresh enable signalTRREN output from the mode register 27 is activated. Therefore, thestate signal TRRST1 becomes the high level.

In this state, a first target-row refresh operation is carried out in abelow manner. First, the target address, in other words, the row addressof the normal word line WLi, in other words, the target address(described as “Target Row” in FIG. 17) is input together from an activecommand ACT from the controller (time t12). As a result, the commanddecoder 25 activates the active signal iACT. Therefore, the row controlcircuit 28 executes row access.

Since the state signals TRRST2 and TRRST3 are at the low level at thispoint of time, the selector 31 shown in FIG. 3 selects the row addressRADD1 (target address) input from outside, and this is output as the rowaddress RADD2. The row address RADD2 is input to the address detectioncircuit 35. However, in the present case, the target address is not adefective address, and none of the hit signals HIT0 to HITm aretherefore activated. Moreover, since the normal word line WL indicatedby the target address is not the normal word line WLN adjacent to theredundant-word-line area A2 in the present case, the flag signal FLG1 isalso at the low level. Furthermore, since none of the hit signals HIT0to HITm are activated, the flag signals FLG2 and FLG3 are also at thelow level.

Therefore, the row predecoder 41 and the row decoder 42 select thenormal word line WLi indicated by the target address input from outside,and the normal memory cells MC connected thereto are refreshed. Then,when a precharge command PRE is issued, the precharge signal iPRE isgenerated by the command decoder 25, and the state signal TRRST2 ischanged to the high level (time t13). As a result, the selection signalSEL2 also becomes the high level.

The first target-row refresh operation is completed as described above.In this manner, in the first target-row refresh operation, the normalword line WLi indicated by the target address is selected. FIG. 17 showsthe case in which the target address corresponds to the normal word lineWL2.

Then, a second target-row refresh operation is carried out. First, thetarget address is input again together with an active command ACT fromthe controller (time t14). As a result, the row control circuit 28executes row access again. However, since the state signal TRRST2 is atthe high level at this point of time, the selector 31 selects the rowaddress RADD1 a output from the address control circuit 32 and outputsthat as the row address RADD2. Since the selection signal SEL2 isactivated to the high level, the address control circuit 32 inverts theleast significant bit X0 of the input target address and outputs this asthe row address RADD1 a.

The more significant bits Xn to X1 of the row address RADD2 generated inthis manner excluding the least significant bit X0 are the same as themore significant bits Xn to X1 of the target address. Therefore, as amatter of course, none of the hit signals HIT0 to HITm are activated.

Therefore, the row predecoder 41 and the row decoder 42 select thenormal word line WLi−1 or WLi+1 (for example, the normal word line WL3)adjacent to one side of the normal word line WLi indicated by the targetaddress input from outside, and the normal memory cells MC connectedthereto are refreshed. Then, when a precharge command PRE is issued, thestate signal TRRST3 is changed to the high level (time t15). As aresult, the selection signal SEL3 becomes the high level instead of theselection signal SEL2.

The second target-row refresh operation is completed as described above.In this manner, in the second target-row refresh operation, the normalword line WLi−1 or WLi+1 adjacent to the one side of the normal wordline WLi indicated by the target address is selected. The normal wordline WLi and the normal word line WLi−1 or WLi+1 are the word lines atwhich only the least significant bits X0 are different.

Then, a third target-row refresh operation is carried out. First, thetarget address is input again together with an active command ACT fromthe controller (time t16). As a result, the row control circuit 28executes row access again. However, since the state signal TRRST3 is atthe high level at this point of time, the selector 31 selects the rowaddress RADD1 a output from the address control circuit 32 and outputsthis as the row address RADD2. At this point, the selection signal SEL3is activated to the high level, and all of the flag signals FLAG1 to 3are deactivated to the low level. Therefore, the calculation enablesignal CALEN is activated to the high level. As a result, the addresscontrol circuit 32 increments or decrements the more significant bits Xnto X0 of the input target address, synthesizes this with the invertedleast significant bit X0, and outputs this as the row address RADD1 a.

The more significant bits Xn to X1 of the target address are decrementedby the address control circuit 32 when the least significant bit X0 ofthe target address is “0”. Reversely, the more significant bits Xn to X1of the target address are incremented by the address control circuit 32when the least significant bit X0 of the target address is “1”.Therefore, if the word line indicated by the target address is thenormal word line WL2, the target address is decremented, and the rowaddress RADD2 indicating the normal word line WL1 is generated.

Note that the present example is explained on the assumption that thenormal word line WLi−1 or WLi+1 is not defective, and, therefore,replacement by the redundant word line RWL is not carried out.Therefore, also in the third target-row refresh operation, the hitsignals HIT0 to HITm are not activated.

As a result, the row predecoder 41 and the row decoder 42 select thenormal word line WLi−1 or WLi+1 adjacent to the other side of the normalword line WLi indicated by the target address input from outside, andthe normal memory cells MC connected thereto are refreshed. Then, when aprecharge command PRE is issued, all of the state signals TRRST1 toTRRST3 are changed to the low level (time t17).

The third target-row refresh operation is completed as described above.In this manner, in the third target-row refresh operation, the normalword line WLi−1 or WLi+1 adjacent to the other side of the normal lineWLi indicated by the target address is selected.

In this manner, in the case 1, the normal word line WLi indicated by thetarget address, the normal word line WLi+1 adjacent to the one side ofthe normal word line WLi, and the normal word line WLi+1 adjacent to theother side of the normal word line WLi are sequentially selected. As aresult, the reduced information retention time are recovered.

Note that the above described example is explained on the assumptionthat the normal word lines WLi−1 and WLi+1 are not defective. However,if the normal word line WLi−1 or WLi+1 is defective and replaced by theredundant word line RWL, the redundant word line RWL of a replacementdestination is selected in the third target-row refresh operation.However, this operation is not essential.

[Case 2 (Number 1)]

FIG. 18 is a first timing chart for explaining the target-row refreshoperations in the case 2. The case 2 is that access with respect to thenormal word line WLN adjacent to the redundant-word-line area A2 hasbeen repeated, the target-row refresh operations are executed withrespect to the normal word line WLN-1 and the redundant word line RWL0adjacent thereto. More specifically, this is the case in which the wordline serving as the cause of the target-row refresh operations belongsto the normal-word-line area A1, one of the word lines serving as thetarget of the target-row refresh operation belongs to thenormal-word-line area A1, and the other one belongs to theredundant-word-line area A2. In this case, it is assumed that the normalword line WLN is not defective. Herein, in the case 2, a case in whichthe redundant word line RWL0 is used is explained.

First, when the number of access with respect to the normal word lineWLN exceeds a predetermined value, a target-row refresh command TRR isissued from the external controller (time t21). As a result, thetarget-row refresh enable signal TRREN output from the mode register 27is activated. Therefore, the state signal TRRST1 becomes the high level.

In this state, a first target-row refresh operation is carried out inthe below manner. First, the target address, in other words, the rowaddress of the normal word line WLN is input together with an activecommand from the controller (time t22). The operation related thereto isas explained by using FIG. 17, wherein the normal word line WLNindicated by the target address input from outside is selected, and thenormal memory cells MC connected thereto are refreshed.

However, in the case 2, all of the values indicating the normal wordline WLN by the target address, in other words, the values of Xn to X0are “1”. Therefore, the AND gate circuit 78 contained in the boundarydetection circuit 36 is outputting the high level.

Then, when a precharge command PRE is issued, the state signal TRRST2and the selection signal SEL2 are changed to the high level (time t23).As a result, the latch circuit contained in the boundary detectioncircuit 36 latches the signal at the high level. Therefore, the flagsignal FL.

G1 is activated to the high level.

The first target-row refresh operation is completed as described above.In this manner, in the first target-row refresh operation, the normalword line WLN indicated by the target address is selected.

Then, a second target-row refresh operation is carried out. First, thetarget address is input again together with an active command ACT fromthe controller (time t24). The operation related thereto is as explainedby using FIG. 17, wherein the row address RADD2 having the invertedleast significant bit X0 of the target address input from outside isgenerated, and the normal word line WLN-1 corresponding thereto isselected.

Then, when a precharge command PRE is issued, the state signal TRRST3and the selection signal SEL3 are changed to the high level (time t25).

The second target-row refresh operation is completed as described above.In this manner, in the second target-row refresh operation, the normalword line WLN-1 adjacent to one side of the normal word line WLNindicated by the target address is selected. The normal word line WLNand the normal word line WLN-1 are the word lines at which only theleast significant bits X0 are different.

Then, a third target-row refresh operation is carried out. First, thetarget address is input again together with an active command ACT fromthe controller (time t26). As a result, the row control circuit 28executes row access again. In the present case, since the flag signalFLG1 is activated to the high level at this point of time, thecalculation enable signal CALEN is fixed to the low level. Therefore,even if the state signal TRRST3 is changed to the high level, incrementor decrement by the address control circuit 32 is forbidden, and the rowaddress RADD2 having the inverted least significant bit X0 is output.

Since the normal word line WLN is not defective as described above, thehit signal HITa shown in FIG. 12 remains at the low level. Then, sincethe redundant word line RWL0 is used in the present example, the enablesignal AFEN0 corresponding to the fuse et PSET0 is at the high level.Therefore, all of the inputs of the AND gate circuit 86 shown in FIG. 12become the high level. Therefore, the usage detection signal USE0becomes the high level. As a result, all of the usage detection signalUSE0, the flag signal FLG1, and the state signal TRRST3 become the highlevel. Therefore, the address conversion circuit 43 shown in FIG. 15activates the switch signal FRC0. As a result, regardless of the inputrow address RADD2, the redundant word line RWL0 is forcibly selected.

Then, when a precharge command is issued, all of the state signalsTRRST1 to TRRST3 are changed to the low level (time t27).

The third target-row refresh operation is completed as described above.In this manner, in the third target-row refresh operation, the redundantword line RWL0 adjacent to the other side of the normal word line WLNindicated by the target address is selected.

If the redundant word line RWL0 is used in the case 2 in this manner,the normal word line WLN indicated by the target address, the normalword line WLN-1 adjacent to the one side of the normal word line WLN,and the redundant word line RWL0 adjacent to the other side of thenormal word line WLN are sequentially selected.

[Case 2 (Number 2)]

FIG. 19 is a second timing chart for explaining the target-row refreshoperations in the case 2. Herein, in the case 2, a case in which theredundant word line RWL0 is unused is explained.

The operations corresponding to time t31 to t35 are similar to those ofFIG. 18. More specifically, the normal word line WLN is selected in afirst target-row refresh operation, and the normal word line WLN-1 isselected in a second target-row refresh operation.

Then, the target address is input in relation to a third target-rowrefresh operation (time t36). In the present example, since theredundant word line RWL0 is unused, the enable signal AFEN0corresponding to the fuse set FSET0 is at the low level. As a result,the usage detection signal USE0 maintains the low level. Therefore,forcible address conversion by the address conversion circuit 43 is notcarried out. In this case, also in the third target-row refreshoperation, the normal word line WLN-1 is selected as well as the secondtime.

In this manner, if the redundant word line RWL0 is unused in the case 2,the normal word line WLN indicated by the target address and the normalword line WLN-1 adjacent to the one side of the normal word line WLN aresequentially selected, while the redundant word line RWL0 is notselected. As a result, unexpected erroneous operations caused byselecting the unused redundant word line RWL0 are prevented.

[Case 3]

FIG. 20 is a timing chart for explaining the target-row refreshoperations in the case 3. The case 3 is that access with respect to theredundant word line RWL0 adjacent to the normal-word-line area A1 hasbeen repeated, the target-row refresh operations are executed withrespect to the normal word line WLN and the redundant word line RWL1adjacent thereto. More specifically, this is the case in which the wordline serving as the cause of the target-row refresh operations belongsto the redundant-word-line area A2, one of the word lines serving as thetargets of the target-row refresh operations belongs to thenormal-word-line area A1, and the other one belongs to theredundant-word-line area A2.

First, a target-row refresh command TRR is issued (time t41), and thestate signal TRRST1 becomes the high level. Then, the address of adefective word line WLt, which is the replacement origin of theredundant word line RWL0, is input as the target address from thecontroller (time t42).

Since the state signals TRRST2 and TRRST3 are at the low level at thispoint of time, the selector 31 shown in FIG. 3 selects the row addressRADD1 (target address) input from outside and outputs this as the rowaddress RADD2. The row address RADD2 is input to the address detectioncircuit 35. However, in the present case, the target address is adefective address, and, the hit signal HIT0 is activated.

Herein, since the least significant bit X0 of the target address is “0”,the row predecoder 41 and the row decoder 42 select the redundant wordline RWL0 based on the hit signal HIT0 and the least significant bit X0of the target address. Then, when a precharge command PRE is issued, thestate signal TRRST2 and the selection signal SEL2 are changed to thehigh level (time t43).

The first target-row refresh operation is completed as described above.In this manner, in the first target-row refresh operation, the redundantword line RWL0, which is the replacement destination, is selected. Notethat, even when the hit signal HIT0 is activated to the high level, ifthe least significant bit X0 of the row address RADD2 is “0”, the usagedetection signal USE0 is not activated by the circuit configuration showin FIG. 12.

Then, a second target-row refresh operation is carried out. First, thetarget address is input again together with an active command ACT fromthe controller (time t44). As a result, the row control circuit 28executes row access again. However, since the state signal TRRST2 is atthe high level at this point of time, the selector 31 selects the rowaddress RADD1 a output from the address control circuit 32 and outputsthat as the row address RADD2. Since the selection signal SEL2 isactivated to the high level, the address control circuit 32 inverts theleast significant bit X0 of the input target address, and this is outputas the row address RADD1 a.

The more significant bits Xn to X1 of the row address RADD2 generated inthis manner excluding the least significant bit X0 are the same as themore significant bits Xn to X1 of the target address. Therefore, as amatter of course, the hit signal HIT0 is activated.

Therefore, the row predecoder 41 and the row decoder 42 select theredundant word line RWL1 adjacent to one side of the redundant word lineRWL0 based on the hit signal HIT0 and the least significant bit X0 ofthe target address. Then, when a precharge command is issued, the statesignal TRRST3 and the selection signal SEL3 are changed to the highlevel (time t45). The second target-row refresh operation is completedas described above. In this manner, in the second target-row refreshoperation, the redundant word line RWL1 in one side of the redundantword line RWL0, which is the replacement destination, is selected. Theredundant word line RWL0 and the redundant word line RWL1 are the wordlines at which only the least significant bits X0 are different.

Since the boundary detection circuit 40 latches the high-level hitsignal HIT0 in response to activation of the state signal TRRST3, theflag signal FLG2 is changed to the high level.

Then, the target address is input in relation to a third target-rowrefresh operation (time t46). However, since the flag signal FLG2 is atthe high level in the present example, all of the values of Xn to X0 ofthe row address RADD1 a output from the address control circuit 32become “1”. The word line at which all of the values of Xn to X0 are “1”corresponds to the normal word line WLN shown in FIG. 2. As a result, inthe third target-row refresh operation, regardless of the input targetaddress, the normal word line WLN is selected. Note that the outputsignal of the AND gate circuit 78 contained in the boundary detectioncircuit 36 is at the high level. However, since the slate signal TRRST2is already at the high level at this point of time, the latch circuit 79does not carry out a latch operation. Therefore, the flag signal FLG1 ismaintained at the low level.

In this manner, in the case 3, the redundant word line RWL0, which isthe replacement destination, the redundant word line RWL1 adjacent tothe one side of the redundant word line RWL0, and the normal word lineWLN adjacent to the other side of the redundant word line RWL0 aresequentially selected.

Note that, the above described example has been explained on theassumption that the normal word line WLN is not defective. However, ifthe normal word line WLN is defective and replaced by the redundant wordline RWL, the redundant word line RWL, which is the replacementdestination, is selected in the third target-row refresh operation.However, this operation is not essential.

[Case 4 (Number 1)]

FIG. 21 is a first timing chart for explaining target-row refreshoperations in the case 4. The case 4 is that access with respect to theredundant word line RWLj (for example, the redundant word line RWL2shown in FIG. 2) is repeated, the target-row refresh operations areexecuted with respect to redundant word lines RWLj−1 and RWLj+1 (forexample, RWL1, RWL3) adjacent thereto. In other words, this is a case inwhich both of the word line serving as the cause of the target-rowrefresh operations and the word lines serving as the targets of thetarget-row refresh operations belong to the redundant-word-line area A2.Herein, in the case 4, a case in which both of the redundant word linesRWLj−1 and RWLj+1 are used is explained.

First, a target-row refresh command TRR is issued (time t51), and thestate signal TRRST1 becomes the high level. Then, the address of adefective word line WLt, which is the replacement origin of theredundant word line RWLj (for example, the redundant word line RWL2), isinput as a target address from the controller (time t52).

Since the state signals TRRST2 and TRRST3 are at the low level at thispoint of time, the selector 31 shown in FIG. 3 selects the row addressRADD1 (target address) input from outside and outputs this as the rowaddress RADD2. The row address RADD2 is input to the address detectioncircuit 35, and, since the target address is a defective address in thepresent case, the hit signal HITk is activated. As a result, the rowpredecoder 41 and the row decoder 42 select the redundant word line RWLjbased on the hit signal HITk and the least significant bit X0 of thetarget address. Then, when a precharge command is issued, the statesignal TRRST2 and the selection signal SEL2 are changed to the highlevel (time t53).

The first target-row refresh operation is completed as described above.In this manner, in the first target-row refresh operation, the redundantword line RWLj, which is a replacement destination, is selected. Theredundant word line RWLj, which is the replacement destination, is, forexample, the redundant word line RWL2.

Herein, the hit signal HITk is any of the hit signals HIT1 to HITm.Therefore, it is not the hit signal HIT0. For example, if the redundantword line RWLj is the redundant word line RWL2, the hit signal HIT1 isactivated. In this case, the least significant bit X0 of the targetaddress is “0”.

Then, in the present example, since the redundant word lines RWLj−1 andRWLj+1 adjacent to the redundant word line RWLj are used, the enablesignal AFENk−1 or AFENk+1 at the high level is input to the detectioncircuit DCk shown in FIG. 11. Therefore, the detection circuit DCkactivates the usage detection signal USEk to the high level, and this islatched in response to the state signal TRRST2. Thus, the flag signalFLG3 becomes the high level.

Then, a second target-row refresh operation is carried out. First, thetarget address is input again together with an active command ACT fromthe controller (time t54). As a result, the row control circuit 28executes row access again. However, since the state signal TRRST2 is atthe high level at this point of time, the selector 31 selects the rowaddress RADD1 a output from the address control circuit 32 and outputsthat as the row address RADD2. Since the selection signal SEL2 has beenactivated to the high level, the address control circuit 32 inverts theleast significant bit X0 of the input target address and outputs this asthe row address RADD1 a.

The more significant bits Xn to X1 of the row address RADD2 generated inthis manner excluding the least significant bit X0 are the same as themore significant bits Xn to X1 of the target address. Therefore, as amatter of course, the hit signal HITk is activated.

Therefore, the row predecoder 41 and the row decoder 42 select theredundant word line RWLj−1 or RWLj+1 (for example, the redundant wordline RWL3) adjacent to one side of the redundant word line RWLj based onthe hit signal HITk and the least significant bit X0 of the targetaddress. Then, when a precharge command PRE is issued, the state signalTRRST3 and the selection signal SEL3 are changed to the high level (timet55).

The second target-row refresh operation is completed as described above.In this manner, in the second target-row refresh operation, theredundant word line RWLj−1 or RWLj+1 adjacent to one side of theredundant word line RWLj, which is the replacement destination, isselected. The redundant word line RWLj and the redundant word lineRWLj−1 or RWLj+1 are the word lines at which only the least significanthit X0 is different.

Then, a third target-row refresh operation is carried out. First, thetarget address is input again together with an active command ACT fromthe controller (time t56). As a result, the row control circuit 28executes row access again. In the present case, since the flag signalFLG3 is activated to the high level at this point of time, thecalculation enable signal CALEN is fixed to the low level. Therefore,even if the state signal TRRST3 is changed to the high level, theincrement or decrement by the address control circuit 32 is forbidden,and the row address RADD2 having the inverted least significant bit X0is output.

Then, in the present example, since the flag signal FLG3 is at the highlevel and the flag signal FLG1 is at the low level, the addressconversion circuit 44 shown in FIG. 16 activates the switch signal FRC1.As a result, the row decoder 42 forcibly selects the redundant word lineRWL which is in the opposite side of the redundant word line RWL havingthe different least significant bit X0 of the row address among theredundant word lines RWL adjacent to the redundant word line RWLj. Forexample, if the input address is indicating the redundant word lineRWL2, the redundant word line RWL1 is forcibly selected. As a result,the redundant word line RWLj−1 or RWLj+1 adjacent to the other side ofthe redundant word line RWLj is selected.

Then, when a precharge command PRE is issued, all the state signalsTRRST1 to TRRST3 are changed to the low level (time t57).

The third target-row refresh operation is completed as described above.

In this manner, if the adjacent redundant word lines RWLj−1 and RWLj+1are used in the case 4, the redundant word line RWLj, which is thereplacement origin, the redundant word line RWLj−1 adjacent to the oneside of the redundant word line RWLj, and the normal word line WLj+1adjacent to the other side of the redundant word line RWLj aresequentially selected.

[Case 4 (Number 2)]

FIG. 22 is a second timing chart for explaining target-row refreshoperations in the case 4. Herein, in the case 4, a case in which theredundant word line RWLj−1 or RWLj+1 is unused will be explained.

The operations corresponding to time t61 to t65 are similar to those ofFIG. 21. More specifically, the redundant word line RWLj is selected ina first target-row refresh operation, and the redundant word line RWLj−1or RWLj+1 is selected in a second target-row refresh operation. Then,the target address is input in relation to a third target-row refreshoperation (time t66). In the present example, the redundant word lineRWLj−1 or RWLj+1 is unused; therefore, it is assumed that the enablesignal AFENk−1 is at the low level if the least significant bit X0 ofthe row address is “0”, and the enable signal AFENk+1 is at the lowlevel if the least significant bit X0 of the row address is “1”.Therefore, the usage detection signal USEk maintains the low level.Therefore, even if the state signal TRRST2 is activated, the flag signalFLG3 is maintained at the low level. Therefore, forcible addressconversion by the address conversion circuit 44 is not carried out, and,also in the third target-row refresh operation, the redundant word lineRWLj−1 or RWLj+1 is selected again as well as the second one.

If the redundant word line RWLj−1 or RWLj+1 is unused in the case 4 inthis manner, the redundant word line RWLj, which is the replacementdestination, and the redundant word line RWLj−1 or RWLj+1 adjacent tothe one side of the redundant word line RWLj are sequentially selected,while the redundant word line RWLj−1 or RWLj+1 adjacent to the otherside is not selected. As a result, unexpected erroneous operationscaused by selecting the unused redundant word line RWL are prevented.

As explained above, in the semiconductor device 10 according to thepresent embodiment, the target-row refresh operations are carried out inconsideration of the presence of the redundant word line. Therefore,even if micro short-circuit is present in the unused redundant word lineor the memory cell(s) connected thereto, the redundant word line likethis is prevented from being selected. As a result, correct target-rowrefresh operations can be realized.

Hereinabove, the preferred embodiment of the present invention has beenexplained. However, the present invention is not limited to the abovedescribed embodiment, various modifications can be made within a rangenot departing from the gist of the present invention, and it goeswithout saying that they are also included in the range of the presentinvention.

What is claimed is:
 1. A method comprising: receiving a first command toenter a target-row refresh mode; receiving a second command and a thirdcommand during the target-row refresh mode, wherein the second commandis accompanied by first address information designating a first wordline of a plurality of word lines in a memory array, wherein the thirdcommand is accompanied by the first address information designating thefirst word line, and wherein the plurality of word lines furtherincludes a second word line that is proximate to the first word line;performing, responsive to the second command and the third command, arefresh operation on the memory array; wherein the performing a refreshoperation on the memory array comprises: responsive to the secondcommand, activating the first word line, and responsive to the thirdcommand, activating the second word line if the second word line isfunctional and not activating the second word line if the second wordline is not functional; and receiving a precharge command between thesecond command and the third command.
 2. The method of claim 1, furthercomprising: before entering the target-row refresh mode, storingdefective address information; wherein the performing a refreshoperation on the memory array further comprises: responsive, at least inpart, to the first address information accompanying the third command,providing second address information designating the second word line,and comparing the second address information with the defective addressinformation to cause the second word line to be activated when thesecond address information does not match the defective addressinformation as the second word line being functional and to be notactivated when the second address information matches the defectiveaddress information as the second word line being not functional.
 3. Themethod of claim 2, wherein each of the first word line and the secondword line is a normal word line; and wherein the plurality of word linesfurther includes a redundant word line that is to be used in place ofthe second word line when the second word line is defective.
 4. Themethod of claim 3, wherein the performing a refresh operation on thememory array further comprises: activating the redundant word line whenthe second address information matches the defective address informationas the second word line being defective.
 5. The method of claim 1,wherein the first word line is a normal word line and the second wordline is a redundant word line; wherein the method further comprises,before entering the target-row refresh mode, storing enable informationindicating whether the redundant word line is used or not used; andwherein the performing a refresh operation on the memory array furthercomprises investigating the enable information to cause the redundantword line to be activated when the enable information indicates that theredundant word line is used as the second word line being functional andto be not activated when the enable information indicates that theredundant word line is not used as the second word line being notfunctional.
 6. The method of claim 1, wherein the first word line is afirst redundant word line and the second word line is a second redundantword line; wherein the method further comprises, before entering thetarget-row refresh mode, storing enable information indicating whetherthe second redundant word line is used or not used; and wherein theperforming a refresh operation on the memory array further comprisesinvestigating the enable information to cause the second redundant wordline to be activated when the enable information indicates that thesecond redundant word line is used as the second word line beingfunctional and to be not activated when the enable information indicatesthat the second redundant word line is not used as the second word linebeing not functional.
 7. A method comprising: receiving a first commandto enter a target-row refresh mode; receiving a second command and athird command during the target-row refresh mode, wherein the secondcommand is accompanied by first address information designating a firstword line of a plurality of word lines in a memory array, wherein thethird command is accompanied by the first address informationdesignating the first word line, wherein the plurality of word linesfurther includes a second word line and a third word line, wherein thesecond word line is proximate to the first word line, and wherein thethird word line is proximate to the first word line so that the firstword line is sandwiched between the second word line and the third wordline; performing, responsive to the second command and the thirdcommand, a refresh operation on the memory array; wherein the performinga refresh operation on the memory array comprises: responsive to thesecond command, activating the second word line if the second word lineis functional and not activating the second word line if the second wordline is not functional, and responsive to the third command, activatingthe third word line if the third word line is functional and notactivating the third word line if the third word line is not functional;and receiving a fourth command during the target-row refresh mode,wherein the fourth command is accompanied by the first addressinformation designating the first word line; and wherein the performinga refresh operation on the memory array further comprises, responsive tothe fourth command, activating the first word line.
 8. The method ofclaim 7, wherein the fourth command is received before receiving each ofthe second command and the third command.
 9. A method comprising:receiving a first command to enter a target-row refresh mode; receivinga second command and a third command during the target-row refresh mode,wherein the second command is accompanied by first address informationdesignating a first word line of a plurality of word lines in a memoryarray, wherein the third command is accompanied by the first addressinformation designating the first word line, wherein the plurality ofword lines further includes a second word line and a third word line,wherein the second word line is proximate to the first word line, andwherein the third word line is proximate to the first word line so thatthe first word line is sandwiched between the second word line and thethird word line; and performing, responsive to the second command andthe third command, a refresh operation on the memory array; wherein theperforming a refresh operation on the memory array comprises: responsiveto the second command, activating the second word line if the secondword line is functional and not activating the second word line if thesecond word line is not functional, and responsive to the third command,activating the third word line if the third word line is functional andnot activating the third word line if the third word line is notfunctional, wherein the first word line, the second word line and thethird word line are a first normal word line, a second normal word lineand a third normal word line, respectively; wherein the method furthercomprises, before entering the target-row refresh mode, storing aplurality of defective address information; and wherein the performing arefresh operation on the memory array further comprises: responsive, atleast in part, to the first address information accompanying the secondcommand, providing second address information designating the secondnormal word line, comparing the second address information with each ofthe plurality of defective address information to cause the secondnormal word line to be activated when the second address informationdoes not match any one of the plurality of defective address informationas the second word line being functional and to be not activated whenthe second address information matches any one of the plurality ofdefective address information as the second word line being notfunctional, responsive, at least in part, to the first addressinformation accompanying the third command, providing third addressinformation designating the third normal word line, and comparing thethird address information with each of the plurality of defectiveaddress information to cause the third normal word line to be activatedwhen the third address information does not match any one of theplurality of defective address information as the third word line beingfunctional and to be not activated when the third address informationmatches any one of the plurality of defective address information as thethird word line being not functional.
 10. The method of claim 9, whereinthe plurality of word lines further includes a first redundant word lineand a second redundant word line, the first redundant word line to beused in place of the second normal word line when the second word lineis defective, and the second redundant word line to be used in place ofthe third normal word line when the third word line is defective; andwherein the performing a refresh operation on the memory array furthercomprises: activating the first redundant word line when the secondaddress information matches any one of the plurality of defectiveaddress information as the second word line being not functional, andactivating the second redundant word line when the third addressinformation matches any one of the plurality of defective addressinformation as the third word line being not functional.
 11. A methodcomprising: receiving a first command to enter a target-row refreshmode; receiving a second command and a third command during thetarget-row refresh mode, wherein the second command is accompanied byfirst address information designating a first word line of a pluralityof word lines in a memory array, wherein the third command isaccompanied by the first address information designating the first wordline, wherein the plurality of word lines further includes a second wordline and a third word line, wherein the second word line is proximate tothe first word line, and wherein the third word line is proximate to thefirst word line so that the first word line is sandwiched between thesecond word line and the third word line; and performing, responsive tothe second command and the third command, a refresh operation on thememory array; wherein the performing a refresh operation on the memoryarray comprises: responsive to the second command, activating the secondword line if the second word line is functional and not activating thesecond word line if the second word line is not functional, andresponsive to the third command, activating the third word line if thethird word line is functional and not activating the third word line ifthe third word line is not functional; wherein the first word line, thesecond word line and the third word line are a first normal word line, asecond normal word line and a redundant word line, respectively; whereinthe method further comprises, before entering the target-row refreshmode, storing a plurality of defective address information and furtherstoring enable information, the enable information indicating whetherthe redundant word line is used or not used; and wherein the performinga refresh operation on the memory array further comprises: responsive,at least in part, to the first address information accompanying thesecond command, providing second address information designating thesecond normal word line, comparing the second address information witheach of the plurality of defective address information to cause thesecond normal word line to be activated when the second addressinformation does not match any one of the plurality of defective addressinformation as the second word line being functional and to be notactivated when the second address information matches any one of theplurality of defective address information as the second word line beingnot functional, and responsive to the third command, investigating theenable information to cause the redundant word line to be activated whenthe enable information indicates that the redundant word line is used asthe third word line being functional and to be not activated when theenable information indicates that the first redundant word line is notused as the third word line being not functional.
 12. The method ofclaim 11, wherein the performing a refresh operation on the memory arrayfurther comprises: when the enable information indicates that theredundant word line is not used, activating the second normal word lineagain if the second normal word line has been activated responsive tothe second command.
 13. A method comprising: receiving a first commandto enter a target-row refresh mode; receiving a second command and athird command during the target-row refresh mode, wherein the secondcommand is accompanied by first address information designating a firstword line of a plurality of word lines in a memory array, wherein thethird command is accompanied by the first address informationdesignating the first word line, wherein the plurality of word linesfurther includes a second word line and a third word line, wherein thesecond word line is proximate to the first word line, and wherein thethird word line is proximate to the first word line so that the firstword line is sandwiched between the second word line and the third wordline; and performing, responsive to the second command and the thirdcommand, a refresh operation on the memory array; wherein the performinga refresh operation on the memory array comprises: responsive to thesecond command, activating the second word line if the second word lineis functional and not activating the second word line if the second wordline is not functional, and responsive to the third command, activatingthe third word line if the third word line is functional and notactivating the third word line if the third word line is not functional;wherein the first word line, the second word line and the third wordline are a first redundant word line, a second redundant word line and anormal word line, respectively; wherein the method further comprises,before entering the target-row refresh mode, storing a plurality ofdefective address information and further storing enable information,the enable information indicating whether the second redundant word lineis used or not used; and wherein the performing a refresh operation onthe memory array further comprises: responsive to the second command,investigating the enable information to cause the second redundant wordline to be activated when the enable information indicates that thesecond redundant word line is used as the second word line beingfunctional and to be not activated when the enable information indicatesthat the second redundant word line is not used as the second word linebeing not functional, responsive, at least in part, to the first addressinformation accompanying the third command, providing second addressinformation designating the normal word line, and comparing the secondaddress information with each of the plurality of defective addressinformation to cause the normal word line to be activated when thesecond address information does not match any one of the plurality ofdefective address information as the third word line being functionaland to be not activated when the second address information matches anyone of the plurality of defective address information as the third wordline being not functional.
 14. A method comprising: receiving a firstcommand to enter a target-row refresh mode; receiving a second commandand a third command during the target-row refresh mode, wherein thesecond command is accompanied by first address information designating afirst word line of a plurality of word lines in a memory array, whereinthe third command is accompanied by the first address informationdesignating the first word line, wherein the plurality of word linesfurther includes a second word line and a third word line, wherein thesecond word line is proximate to the first word line, and wherein thethird word line is proximate to the first word line so that the firstword line is sandwiched between the second word line and the third wordline; and performing, responsive to the second command and the thirdcommand, a refresh operation on the memory array; wherein the performinga refresh operation on the memory array comprises: responsive to thesecond command, activating the second word line if the second word lineis functional and not activating the second word line if the second wordline is not functional, and responsive to the third command, activatingthe third word line if the third word line is functional and notactivating the third word line if the third word line is not functional;wherein the first word line, the second word line and the third wordline are a first redundant word line, a second redundant word line and athird redundant word line, respectively; wherein the method furthercomprises, before entering the target-row refresh mode, storing firstenable information and second enable information, the first enableinformation indicating whether the second redundant word line is used ornot used, and the second enable information indicating whether the thirdredundant word line is used or not used; and wherein the performing arefresh operation on the memory array further comprises: responsive tothe second command, investigating the first enable information to causethe second redundant word line to be activated when the first enableinformation indicates that the second redundant word line is used as thesecond word line being functional and to be not activated when the firstenable information indicates that the second redundant word line is notused as the second word line being not functional, and responsive to thethird command, investigating the second enable information to cause thethird redundant word line to be activated when the second enableinformation indicates that the third redundant word line is used as thethird word line being functional and to be not activated when the secondenable information indicates that the third redundant word line is notused as the third word line being not functional.
 15. An apparatuscomprising: a memory array comprising a plurality of word lines, theplurality of word lines including first, second and third word lines,the second word line being proximate to the first word line, and thethird word line being proximate to the first word line so that the firstword line is sandwiched between the second word line and the third wordline; a command/address circuit configured to, during a target-rowrefresh mode, first, second and third commands, each of the first,second and third commands being accompanied by first address informationdesignating the first word line; and a refresh control circuitconfigured to: responsive to the first command, activate the first wordline, responsive to the second command, activate the second word line ifthe second word line is functional and not activate the second word lineif the second word line is not functional, and responsive to the thirdcommand, activate the third word line if the third word line isfunctional and not activate the third word line if the third word lineis not functional; wherein the first word line, the second word line andthe third word line are a first normal word line, a second normal wordline and a third normal word line, respectively; wherein the apparatusfurther comprises: a defective address storage circuit configured tostore a plurality of defective address information; and a refreshaddress circuit configured to provide second address informationdesignating the second normal word line responsive to the first addressinformation accompanying the second command, the refresh address circuitfurther configured to provide third address information designating thethird normal word line responsive to the first address informationaccompanying the third command; and wherein the refresh control circuitis further configured to: compare the second address information witheach of the plurality of defective address information to cause thesecond normal word line to be activated when the second addressinformation does not match any one of the plurality of defective addressinformation and to be not activated when the second address informationmatches any one of the plurality of defective address information, andcompare the third address information with each of the plurality ofdefective address information to cause the third normal word line to beactivated when the third address information does not match any one ofthe plurality of defective address information and to be not activatedwhen the third address information matches any one of the plurality ofdefective address information.
 16. An apparatus comprising: a memoryarray comprising a plurality of word lines, the plurality of word linesincluding first, second and third word lines, the second word line beingproximate to the first word line, and the third word line beingproximate to the first word line so that the first word line issandwiched between the second word line and the third word line; acommand/address circuit configured to, during a target-row refresh mode,first, second and third commands, each of the first, second and thirdcommands being accompanied by first address information designating thefirst word line; and a refresh control circuit configured to: responsiveto the first command, activate the first word line, responsive to thesecond command, activate the second word line if the second word line isfunctional and not activate the second word line if the second word lineis not functional, and responsive to the third command, activate thethird word line if the third word line is functional and not activatethe third word line if the third word line is not functional; whereinthe first word line, the second word line and the third word lines are afirst normal word line, a second normal word line and a redundant wordline, respectively; wherein the apparatus further comprises: a defectiveaddress storage circuit configured to store a plurality of defectiveaddress information; an enable information storage circuit configured tostore enable information indicating whether the redundant word line isused or not used; and a refresh address circuit configured to providesecond address information designating the second normal word lineresponsive to the first address information accompanying the secondcommand; and wherein the refresh control circuit is further configuredto: compare the second address information with each of the plurality ofdefective address information to cause the second normal word line to beactivated when the second address information does not match any one ofthe plurality of defective address information and to be not activatedwhen the second address information matches any one of the plurality ofdefective address information, and investigate the enable information tocause the redundant word line to be activated when the enableinformation indicates that the redundant word line is used and to be notactivated when the enable information indicates that the redundant wordline is not used.
 17. An apparatus comprising: a memory array comprisinga plurality of word lines, the plurality of word lines including first,second and third word lines, the second word line being proximate to thefirst word line, and the third word line being proximate to the firstword line so that the first word line is sandwiched between the secondword line and the third word line; a command/address circuit configuredto, during a target-row refresh mode, first, second and third commands,each of the first, second and third commands being accompanied by firstaddress information designating the first word line; and a refreshcontrol circuit configured to: responsive to the first command, activatethe first word line, responsive to the second command, activate thesecond word line if the second word line is functional and not activatethe second word line if the second word line is not functional, andresponsive to the third command, activate the third word line if thethird word line is functional and not activate the third word line ifthe third word line is not functional; wherein the first word line, thesecond word line and the third word line are a first redundant wordline, a second redundant word line and a normal word line, respectively;wherein the apparatus further comprises: a defective address storagecircuit configured to store a plurality of defective addressinformation; an enable information storage circuit configured to storeenable information indicating whether the second redundant word line isused or not used; and a refresh address circuit configured to providesecond address information designating the normal word line responsiveto the first address information accompanying the third command; andwherein the refresh control circuit is further configured to:investigating the enable information to cause the second redundant wordline to be activated when the enable information indicates that thesecond redundant word line is used and to be not activated when theenable information indicates that the second redundant word line is notused, and compare the second address information with each of theplurality of defective address information to cause the normal word lineto be activated when the second address information does not match anyone of the plurality of defective address information and to be notactivated when the second address information matches any one of theplurality of defective address information.
 18. An apparatus comprising:a memory array comprising a plurality of word lines, the plurality ofword lines including first, second and third word lines, the second wordline being proximate to the first word line, and the third word linebeing proximate to the first word line so that the first word line issandwiched between the second word line and the third word line; acommand/address circuit configured to, during a target-row refresh mode,first, second and third commands, each of the first, second and thirdcommands being accompanied by first address information designating thefirst word line; and a refresh control circuit configured to: responsiveto the first command, activate the first word line, responsive to thesecond command, activate the second word line if the second word line isfunctional and not activate the second word line if the second word lineis not functional, and responsive to the third command, activate thethird word line if the third word line is functional and not activatethe third word line if the third word line is not functional: whereinthe first word line, the second word line and the third word line are afirst redundant word line, a second redundant word line and a thirdredundant word line, respectively; wherein the apparatus furthercomprises: an enable information storage circuit configured to storefirst enable information and second enable information, the first enableinformation indicating whether the second redundant word line is used ornot used, and the second enable information indicating whether the thirdredundant information is used or not used; and wherein the refreshcontrol circuit is further configured to: investigate the first enableinformation to cause the second redundant word line to be activated whenthe first enable information indicates that the second redundant wordline is used and to be not activated when the first enable informationindicates that the second redundant word line is not used, andinvestigate the second enable information to cause the third redundantword line to be activated when the second enable information indicatesthat the third redundant word line is used and to be not activated whenthe second enable information indicates that the third redundant wordline is not used.